Grills on AR Mids

[oldguide]

I have recently had to install some replacements for the mids in my AR3. The old mid is the one marked 9a in the manual.

This experience has me wondering about the function and value of the screens and small circular piece of cotton(?) in the old AR3/3a mids. I thought I would test the grill on the new mids to see if there was any difference.

My plan was to anchor it with electrical tape--but that was not a good idea. The issue of where and how "tight" to put the tape made the whole test somewhat ridiculous. At one point I could actually feel the air pulses coming out from under the cotton pad.

However, I am convinced that the grill DID make a difference, but to properly test it, one would have to anchor the grill with the equivalent of the glue of the original and test it with instrument readings.

I should have better checked the previous posts as I see there is a thread going on tweeter grills, but my questions are of a different order. I am curious about why they were there in the first place.

This leads to some questions for some of the old AR hands--did the grills perform any function? What impact did they have on the sound? Why did AR and others abandon the grills? How exactly were they shaped? How much tolerance is there between the grill and the actual driver?

I note on the tweeter thread someone was wondering what the grills were made of. Mine appear to be made of what I know of as "hardware cloth"--at least the grill appears to be the same pattern and resiliency of the HW cloth I have on hand. But shaping them appears a bit tricky. You would almost have to have an old one to match as the distance between driver and grill needs to accommodate the cotton and allow the driver to operate freely. Just throwing a grill on a driver--at least based on my limited experience with the AR3s does not appear a good idea.

Similarly just pulling the grills seems a bit drastic. I pulled mine mainly out of curiosity since the driver was dead I wanted to explore how it was constructed.

[genek]

This leads to some questions for some of the old AR hands--did the grills perform any function? What impact did they have on the sound? Why did AR and others abandon the grills? How exactly were they shaped? How much tolerance is there between the grill and the actual driver?

I think the primary purpose of the grills is just to retain the external damping on the driver, which is there to help "tune" the response. I've been warned over the years by various dealer service people that running the driver at high levels without the external damping may be harmful to the driver's dome, but not sure how reliable that is, since later dome drivers whose response didn't require external damping to tune just did away with it.

The expanded metal would have been shaped by pressing flat stock between mold forms.

[Mexicomike]

According to info from AR at the time, the grill and associated padding was there to attenuate the midrange output. I seem to recall that it also helped dispersion. Doesn't sound too precise, does it? On the other hand, one could argue that IF each speaker was individually assembled/evaluated, a more consistent sound could be obtained. However, personally I doubt if they actually checked each speaker for freq response though I don't know that for sure. I'm sure there are folks here with better specific knowledge - I am remembering this from a phone conversation with an AR tech back in the early 70's when I was returning a 3a for a blown driver for the second or third time!

[genek]

According to info from AR at the time, the grill and associated padding was there to attenuate the midrange output. I seem to recall that it also helped dispersion. Doesn't sound too precise, does it?

What's happening is that attenuating the output at certain angles but not others can "even out" irregular disperson, which is why you see some drivers with a round pad in the center and others with donut-shapes, and pads of varying thickness. If all that's needed is just to reduce overall level, then you end up with a flat disc like the ones used in the 2ax. This isn't something that would necessarily be tweaked on an individual driver basis, but design changes over time that cause output to change would result in periodic configuration changes, which is why there are multiple P/Ns with slightly different-appearing bits on them.